You can't use them but you can prepare them for the eventual war. They will automatically activate on August 1st 1945 and before that if the Japanese crosses the border, attacks or let the garrison fall below a certain amount in Manchukuo. Check the manual, chapter 8.6 for the official description of this.

Considering the increase in bases, units and things to do I would love to have the zone control feature back. Definately making me do my homework and plan way more. Any chance it might come back in the future?